We propose to develop two assays that should be useful for clinical diagnosis of hypercholesterolemias. The first is an assay for familial hypercholesterolemia. It is based on the quantitation of low density lipoprotein (LDL) receptor levels in peripheral blood lymphocytes. LDL receptor expression will be maximally induced by depletion of cellular cholesterol, followed by quantitation of receptor number using flow cytometry utilizing a sandwich antibody technique. The assay should be sufficiently simple and inexpensive to allow rapid screening for the disease, which occurs at a frequency of about 1/500 in the population. The second is a direct assay for LDL cholesterol. It is based on a simple lipoprotein fractionation scheme involving removal of the other lipoprotein species, VLDL and HDL, using specific antibodies on magnetic beads. The LDL cholesterol remaining in solution can now be directly quantitated by standard methods. The assays should be useful for clinical analysis and diagnosis of hyperlipidemias and evaluation of risk of cardiovascular disease.